My girlfriend wants an external for backing up photo's music on her I-mac (no e-sata hookup). I will be building new system (running XP) in the months to come. Would this be a good option? Can I share the hard drive, and if so would I have to partition into two diff hd's?

not too up on mac's myself, but i would venture a guess to share the drive between the i-mac and PC, the hdd would have to be split into at least two partitions. def go with the e-sata (it has USB 2.0 as well) since it is just as fast as internal sata.